Why are we so afraid of aliens

Why is it that almost every time you see a movie or read a book about extraterrestrial beings, they are evil and dangerous?
Why can't there be friendly aliens? You know, the kind that only wants the human species on this little out-of-the-way planet to join the community of star travelers.
Would it be too much to ask people to believe that other forms of life in the universe are friendly and pose no danger to humanity?
I think the reason is pretty simple, yet very hard to combat. We, all of us, fear things we do not understand. The natural reaction is to get defensive and to stir up that fear so we will have the motivation to take action to protect ourselves.
It makes sense. Better to be prepared than to remain unprepared and vulnerable. Even if we knew that at some defined point in the near future we would have to say hello to an alien, would we do as much to prepare if we had nothing to fear?
I doubt it.
So, we use fear to motivate us. The fear, I believe is innate. It comes from generations and generations of having to fight to survive. If our ancestors did not fight, they would be killed, by an animal, by another human, by nature. It truly was survival of the fittest and we used fear to keep our fighting edge honed.
Sitting in our cave, or in some hut in Africa, we learned to fear the night, fear big animals, fear noises that threatened us, fear different peoples, fear things we didn't understand. Fear has kept us alive, allowed our species to become the most dominate on this planet.
But what about other planets? We keep on discovering new planets outside our solar system and someday we will find one that supports life. I likely will be long dead before we find one that supports intelligent life or a species that has developed any kind of technology, especially a technology that will allow us to communicate with them.
I just hope that when that discovery comes, we can put our fear aside long enough to not be immediately hostile to those communicating with us. We need to be prepared. But we need to be peaceful, have an open mind and look for the best in whatever we face.
Science fiction is full of scary stuff -- intelligent species that are very ugly and mean, usually in the form of some giant thing that in its smaller form we are afraid of, like spiders or worms, bees or ugly reptiles.
We don't often imagine aliens being like teddy bears or pretty ponies or mermaids. Friendly things just don't fit with our concept of the unknown.
Fear has got a grip on us. I just hope we can pry loose of that by the time we do have to meet our neighbors from the universe.